Use one graph-paper rectangle to trace a rectangle on the stiff paper. Arrange it so that you've left room for a top and bottom and side flaps.
Now fold the graph paper in half. You'll be measuring to make the top flap, but you'll do half of it in order to get an exact mirror image.
Draw dots about a half inch (for a small envelope) or whole inch (for a bigger envelope) from the corners. The slant of the top flap is more slanted near the corners of the envelope. The dot will mark where the slant changes.
Connect the dots using a paper edge to make your line straight.
Sketch out the shape, connecting the straight line to the corners, rounding slightly where the dots are. These rounded edges are what will help make the envelope look nice.
Cut along the line, and then unfold. This symetrical triangular-shape is what you'll trace to make the top flap.
Trace its shape in three places: where the flap goes on the stiff paper, inside the rectangle (to help visualize how the flaps look as you go), and on the other piece of graph rectangle to help show where to measure for the other flaps.
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